# index, an index or the index?

Context: Imagine a sequence of indices greater than 1. The sequence must follow these rules:

1.) Index 2 cannot be surrounded by two indices 3.

...

(I know this may be written in much more simple way, but the structure should remain the same.)

The question: in the beginning of the first rule, should there be any article before the word "Index" (like: The index 2... / An index 2...) or is it also correct without any article?

Is "Index 2" in this context definite or not? (there may be more indices 2 in the sequence and the rule should apply to all of them)

• I'm not sure I understand. What is "Index 2"? How can there be multiple of them (unless we're talking CS, and they're from multiple arrays, in which case they wouldn't be surrounded by two "indices 3")? Typically "the index" would be used when referring to a specific index, and "an index" would be used to refer to a generic index.
– Doc
Feb 18 '14 at 16:32
• @Doc Maybe I haven't used the right expression, What I meant was sequence of numbers like: 2,2,5,4,6 where 3,2,3 would be against the rule 1. What word would better describe a member of the sequence than "index" in this case?
– Riko
Feb 18 '14 at 16:33
• "A value of '2' cannot be surrounded by two instances of the value '3'". An index is the position in the list, the value is the member at that index. So for a list of: {1,2,3,4,5}; the value at index 0 is 1, the value at index 2 is 3, etc (or if index values start at 1, then the value at index 1 is 1 and the value at index 3 is 3).
– Doc
Feb 18 '14 at 16:38

If there can be more than one instance of an item in a given set, and you are not limiting your comment or rule to only one of those instances, the reference is indefinite. You should use an indefinite article (not a definite article and not no article) when referring to one instance of an item.

You could say

An Index 2 cannot be surrounded by two Index 3s.

The Index 2 ... and Index 2 ... both suggest that you are only talking about one instance.

But if you construct your sentence so that you are referring to more than one instance, an would not be appropriate. You could also say

Any one of the Index 2s cannot be surrounded by two Index 3s

In this case, you are actually referring to the whole set of Index 2s and making a rule for each and every one of them.

Also, once you have named a variable such as Index 2, it tends to be treated as a proper noun and it is usually pluralized as a unit. I think you would more often find Index 2s as the plural, rather than Indices 2.

• Thanks for your answer, it makes much more sense to me now. One more related question though: is "Index 2 ..." (with no article) in case you mentioned grammatically correct? Or does every noun in singular have to have an article?
– Riko
Feb 18 '14 at 16:51
• Index 2 (without an article) is fine so long as you are talking in general, such as Index 2 is the category that has the following properties ...
– bib
Feb 18 '14 at 17:01